Nitride semiconductors such as GaN, AlN, and InN and a material made of a mixed crystal of two or more of the nitride semiconductors have a wide band gap and are used, for example, for high-power electronic devices and short-wavelength light-emitting devices. For example, GaN, which is a nitride semiconductor, has a band gap of 3.4 eV that is greater than the band gap 1.1 eV of Si and the band gap 1.4 eV of GaAs.
An example of a high-power electronic device is a field effect transistor (FET), particularly, a high electron mobility transistor (HEMT) (see, for example, Japanese Laid-Open Patent Publication No. 2002-359256). A HEMT including a nitride semiconductor is used, for example, for a high-power, high-efficiency amplifier and a high-power switching device. For example, there exists a HEMT including an electron supply layer made of AlGaN and an electron transit layer made of GaN. In this HEMT, piezoelectric polarization occurs in AlGaN due to a lattice constant difference between AlGaN and GaN and as a result, a highly-concentrated two-dimensional electron gas (2DEG) is generated. For this reason, the HEMT can operate at a high voltage, and can be used, for example, for a high-efficiency switching device and a high-voltage electric device for an electric vehicle.
Also, because a HEMT has excellent high-speed characteristics, its use for a signal processing circuit of an optical communication system and other high-speed digital circuits is being considered. Particularly, due to its low-noise characteristics, there is a demand to use a HEMT for an amplifier in a microwave band or a millimeter wave band. When an amplifier is used in a millimeter wave band, a high current-gain cutoff frequency (fT) is required to achieve a sufficient amplifier gain. For this purpose, it is necessary to improve a mutual conductance (gm) that is a parameter related to an amplification factor of a transistor as well as to reduce the gate-source capacitance by reducing the gate length. Also, when a semiconductor device is implemented as a monolithic microwave integrated circuit (MMIC) to reduce its module size, a parasitic capacitance is generated by an interlayer insulating film between wiring layers and therefore it is desired to reduce the dielectric constant of the interlayer insulating film. For this reason, in a semiconductor device such as an MMIC having a multilayer wiring structure, benzocyclobutene (BCB) or polysilazane is used, for example, to form a low-dielectric-constant insulating film implementing an interlayer insulating film.
A low-dielectric-constant insulating film used as an interlayer insulating film has a low film density and therefore has a low water resistance compared with an insulating film that is formed with, for example, SiO2 or SiN by a general film forming method. For this reason, in a semiconductor device such as an MMIC having a multilayer wiring structure, atmospheric moisture may penetrate through a low-dielectric-constant insulating film and oxidize metal that forms wiring. To prevent the penetration of moisture, a protective film is formed on an exposed part of the low-dielectric-constant insulating film (see, for example, Japanese Laid-Open Patent Publication No. 2003-282698 and Japanese Laid-Open Patent Publication No. 2009-76855). However, a protective film formed by a related-art method may not sufficiently prevent the penetration of moisture. When metal that forms wiring is oxidized, it causes a short circuit between wires, increases contact resistance, and reduces electric characteristics. Thus, penetration of moisture reduces the reliability and life of a semiconductor device.